Heat treatment of ferrous metals with fluidized particles



July 27, 1965 J. c. MUNDAY 3,197,346

HEAT TREATMENT OF FERROUS METALS WITH FLUIDIZED PARTICLES original FiledNov. 27, 195s 2 Sheets'sheet 1 9 .2955..6'2 25 )is flo "1 2| 4 Il 2 r" Ws f @S A. F

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John C. Mundoy Inventor By j Po'fentAf'rorney July 27, 1965 J. c. MUNDAY3,197,346

HEAT TREATMENT OF FERROUS METALS WITH FLUIDIZED PARTICLES original FiledNov. 2?,v 1953 2 Sheets-Sheet 2 0m hm Inventor PoTenf Aforney John C.Munduy By QJ/ZKM7 United States Patent O lohn C. Monday, Cranford, NJ.,assigner to Esso Re- Search and Engineering Company, a corporation ofDelaware @riginal application Nov. 27, 1953, Ser. No. 394,747, nowPatent No. 3,053,794, dated Sept. 1l, i952. Divided and this applicationFell. 27, 1962, Ser. No. 176,606

i3 Claims. (Cl. 148-13) This application is a division of applicationSerial No. 394,747, filed November 27, 1963, now Patent No. 3,053,704,granted September l1, 1962, which in turn is a continuation-impart ofprior application Serial No. 174,636, filed luly 19, 1950, nowabandoned.

The present invention relates to a method and apparatus for heattreating metal objects, wherein the treatment involves heating and/orcooling steps applied primarily for the purpose of conserving in orimparting .to the metal objects certain improved physical properties.The metal objects contemplated may be parts, castings, forgings or thelike which customarily are subjected to some form of heat treatmentduring or subsequent to manufacture of the objects, and these objectsmay be of ferrous or non-ferrous metals and their alloys. The heattreatments contemplated include annealing, hardening, tempering, andquenching in their various forms, and combinations of such treatments,and particularly such treating methods wherein the heating and coolingsteps involved produce changes in the physical characteristics -of themetals. Under such circumstances it is Well known that control of therates of heating and coolin(r of the objects during treatment is ofcritical importance in obtaining the desired results. ln the prior art,however, close and uniform control has been difficult to obtain whenemploying conventional heat treating furnaces and liquid treating baths,ln addition to the problems of process control set forth above,conventional systems according to the prior art are subject to defectsproduced oy the heating and cooling media employed therein, and themetal treated may suffer from cracking, oxidation, scaling or otherundesirable surface injuries.

It is an object of the present invention to provide a method foraccomplishing the heat treatment of metals in which the defects of themethods and systems are overcome to provide close and reproduciblecontrol of temperatures in all stages of the treating operation. lt isanother object of the invention to avoid or substantially reduce anyundesirable surface impairment of the metal objects as a result ofoxidation, scaling and other comparable conditions normal to theprocesses and methods of the prior art.

The present invention also contemplates a system, including a method andapparatus, which is useful in heat treating ferrous metals as employedfor the purpose of carburizing and nitriding such metals, and in thetreatment of metals at high temperatures with high melting cyanides,such as sodium cyanide and potassium cyanide. Still another object ofthe present invention is to provide an improved method and apparatus forheat treatment of metal objects to form thereon an alloy coating ofanother metal, for example for the purpose of coating metal objects withother metals such as aluminum, Zinc, chromium, cadmium, vanadium,cobalt, titanium, silicon, Zirconium, tungsten, molybdenum, boron,manganese, and beryllium.

A particular object or" the invention is to provide a method andapparatus wherein metal objects are subjected to heat treatment in thepresence of and by immersion in and in contact with a body of finelydivided or powdered solid materials maintained in a iluidzed conditionin a confined treating zone, and wherein lluidization of the solidmaterials imparts substantially constant motion to the individualparticles of the solid materials such as to produce impinging Contact ofsuch particles against the metal object when immersed therein. Ascontemplated by the present invention treatment of the metal objects maybe accomplished under optimum conditions such as permit heat transferbetween such objects and the powdered materials at more closelycontrolled rates and while avoiding undesirable side effects. Also ascontemplated by the present invention the physical properties of themetal objects may be modified and determined within closer limits andwith greater reproducible uniformity than has been previously known inthe art.

The invention and its objects may be more fully understood from thefollowing specification when it is read in conjunction with theaccompanying drawings in which:

FIG. 1 is a view in vertical section through one form of apparatusaccording to the present invention;

FlG. 2 is a similar view of another embodiment of the invention takenalong line 2-2 of FIG. 3; and

FIG. 3 is a cross-sectional view of the apparatus according to FIG. l,taken along the line 3 3 thereof.

Referring to the drawings in greater detail, in FlG, l, the numeral 1designates a :treating vessel which contains a bed of finely divided orpowdered solid material indicated by the numeral 2. Disposed within thevessel, in the bottom thereof one or more fluid conduits 3 are providedfor the injection of a gaseous medium upwardly into the bottom portionof the bed of solid materials, to tluidize the bed. The dischargeconduits 3 are counected to a manifold conduit d, which extendsoutwardly through la wall of the vessel into connection with a supplyconduit S. rlhe supply conduit 5 is provided with a control valve 6.

When fluidized, the bed or body of powdered materials is maintained withan upper surface level as indicated by the numeral 'i'. In the vessel'as shown the distance between this upper level 7 and the upper end Iofthe vessel 1 is preferably such that the fine-r particles of the solidm-aterials in the bed which may be entrained by the gaseous fluidizingmedium passing through the surface of the bed substantially are notcarried beyond the upper end of the vessel under operating conditions.This distance is determinable by one skilled in the art on the basis ofthe physical characteristics of the powdered material, including 'thecomposition thereof, the particle size and the density of the material,and the velocity at which the gaseous fluidizing medium passes upwardlythrough the bed. The depth of the bed below the surface level 7 will besuch as to permit complete immersion therein of a metal object to betreated in the vessel.

Also as shown by FIG. 1, means are provided for heating and cooling thebed of solid material, by direct or indirect heat exchange contact withiluid heat eX- change media. Within the vessel l are provided a seriesof heat exchange conduits arranged as a coil circumferentially of thevessel, but spaced from the wall thereof. in the arrangementillustrated, the conduits 8 are connected to a supply conduit 9 for aliquid medium, the conduit g extending into the vessel through a sidewall thereof. Exteriorly of the vessel 1 is a separate vessel liladapted to contain a supplementary body 11 ofthe finely divided solidmaterial forming the bed Z in vessel l. When llnidized, this body of thematerial has an upper surface level as at l2. The bottom of vessel lllis connected to the bottom portion of vessel 1 as by means of a transferconduit 1.3, provided with a control valve le. The vessel lll is alsoconnected to the vessel l by means of a second transfer conduit 15opening from an intermediate or upper level therein, above the upperlevel of bed 2 in vessel 1, into the upper portion of vessel 1 below theupper level of the bed 2. A control valve 16 is provided in the secondtransfer conduit 15. The vessel l@ provides for heating or cooling ofthe iinely divided solid material employed in the system by direct heatexchange between said material and a gaseous heat exchange medium. Asshown, a supply conduit 17 for such medium is extended into the transferconduit 13, having an outlet 1S therein opening in the direction of thevessel 10. If desired a burner device may be substituted for the outlet18, to be supplied with a combustible mixture of fuel and air throughmeans such as the supply conduit 17 shown. A control valve 19 isprovided in the conduit 17.

In the upper end of the vessel 1%, there is provided a suitable type ofseparator element, such as a cyclone separator 23 having a dip leg 21extending downwardly below the level 12 of the body of material 11 inthe vessel 1?. An outlet conduit for the iiuid heat exchange mediumintroduced by way of the conduit 17,`or for combustion gases where aburner is substituted, is designated by the numeral 22. A valve Z3 isprovided in the outlet conduit 22.

A conveyor 24 is provided for carrying a metal object, such as a blockof metal indicated by the numeral 25, into and through the treatingzone. As shown, the conveyor 24 is equipped with dependent carriers suchas the arms 26 pivotally mounted thereon. The conveyor is arranged insuch fashion that as it is moved over the body of tiuidized solidmaterials, the carriers will immerse the metal object supported therebyin the body of solids during the treating operation, and then Withdrawit from the bath and the vessel.

Referring now to the apparatus as illustrated in FIGS. 2 and 3, thenumeral 31 designates another form of 'treating vessel. The vessel 31 isan elongated structure having a roof portion 32 terminating at one endin spaced relation to one end wall 33 of the vessel. A vertical baffle34 is secured to the terminal end of the portion 32., transversely ofthe vessel, so as to'extend upwardly above said portion, and downwardlyto depend therefrom into vertically spaced relation to the bottom wallof the vessel. The end wall 33 and the upwardly extended portion of thebaffle 34, along with conforming side wall portions of the vessel,indicated by the numeral 35, define a well 36 open at its upper end, andin direct communication with the vessel at its lower end.

At the-opposite end of the vessel 31, a transverse partition 37, withthe end wall indicated at 3S, and adjoining side wall portions of thevessel define an enclosed chamber 39 with the roof portion 32 extendedthereover. Between the partition 37 and the baille 34, the vesselprovides an enclosed treating chamber which is designated in thedrawings by the numeral di?. In the apparatus as illustrated by FIG. 2,the chamber d@ is divided longitudinally bymeans of a vertical bafflemember t1 disposed in laterally spaced substantially parallel relationto the side walls of the vessel, and in spaced relation at each end tothe battle 34 and the partition 37, respectively. Preferably the bafieis mounted on the oor or bottom of the vessel extending upwardly intovertically spaced relation to the roof portion 32, and provides a pairof laterally defined substantially continuous travel paths through thechamber 413 on each side of the bafde which pathsl are in communicationat each end. As shown, the spacing of the baille 41 from the partition37 is preferably greater than from the transverse baliie 34.

The chamber 39 and the chamber 4t) are provided for direct communicationwith each other, as by means of a pair of parallel, vertically spaced,slot-like passageways 42 and 43 through the partition 37, and extendingvlongitudinally thereof. Each passageway 42 and d3 is provided with anadjustable valve-like closure plate element, such as the elements d and45 shown. These elements may be mounted as on rotatable shaft supportsze and 47, respectively, extending through the vessel side walls andprovided for operation as by suitable handles or valve wheels asindicated in FIG. 3 by the numeral The vessel 31 contains a body offinely divided or powdered solid material designated in FIG. 2 by thenumeral 49. The material is iluidizable by a gaseous uidizing medium,and when so tluidized will till the vessel, including well 36, chamber39, and chamber to a depth and a level such as indicated by the numeralsSi?, 51, and 52 respectively. The actual depth and therefore the actuallevel which may be attained will be determined substantially in the samemanner and for the same purposes referred to in connection with theapparatus as shown in FG. 1, and as may be described below. in anyevent, it is intended that the level attained in chamber ed at all timeswill be above the lower end of the transverse baille 3ft, and such as toprovide a free space between the body of solid material and the roofportion. Likewise, a free space will be maintained above the level 51 inchamber 39.

Fludization of the body of material in the chamber du and well 36 isaccomplished by means for injecting a gaseous medium such as provided bya series of manifold injection nozzles 53, arranged substantially asshown in FIG. 3, and of which each nozzle manifold is connected to acommon supply conduit 54 as by means of branch lines 55 substantially inthe manner illustrated in FIG. 2. Valves such as indicated in the branchlines S5 may bc employed to control the injection of the uidizing mediumin any desired fashion. The supply conduit 54 is in turn connected to asource of a gaseous uidizing medium as indicated by conduit E6. Theconduit 55 is provided with a pump 57. Flow through conduit 56 may becontrolled as by means of a valve 58 preceding the pump, and byoperation of the pump itself.

The chamber fr@ is provided with suitable means for venting the gaseousliuidizing medium therefrom. This, as shown, includes a conduitconnection 6) opening from chamber d@ into a cyclone separator 61. Theseparator 61 is provided with a dip leg return 62 for solids entrainedby the gaseous medium and a vent line 63 for the gaseous medium. Theline 63 is also connected to the conduit 55 ahead of pump 57 as by meansof a conduit connection 64. Valves 65 and 65 in lines 63 and 6drespectively permit selective disposition of the gaseous medium asvented from the chamber 4t).

In the apparatus as illustrated in FIGS. 2 and 3, the chamber 39 isprovided with means for heating the iinely divided solids contained inthe vessel. As shown, chamber 39 is provided with a` plurality of fuelburner elements 67 disposed in the lower portion thereof. Conduits suchas conduit 68 connect the burners 67 with a source of a combustiblemixture of fuel and air for burning within -the chamber 39. Alternatelythe burners 67 may be eliminated and hot ilue gases or other gases athigh temperatures may be fed through conduit 68 from an exterior source.In either event, the solid material in charnber 39 is liuidized by thegases thus formed or introduced.

The chamber 39 is also provided with means for venting the gaseousmedium passed into heat exchange relation with the solid materialstherein, as by a conduit connection 6i) opening from the chamber 39 intoa cy- .clone separator 7d. A dip leg conduit 71 opens from .the bottomof separator 69 to a level below the surface ,level S1 of the tluidizedsolids in chamber 39, to return solids separated in cyclone 7@ from thegaseous medium therein. A vent line 72 from the separator 7) dischargesthe gaseous medium passed through the separator from y chamber 39.

,shown, the conveyor' enters and leaves the vessel through the well 35.Enteiing the vessel downwardly through well 36, the conveyor extendsunder the baffle 34, through a first travel path toward the partition37, substantially across the face of the partition, and thence extendsthrough the second travel path toward the baie 34, under the bafiie andthen upwardly and out through the well 36. A metal object `such asindicated by the numeral 74 is supported on the conveyor in anyconventional fashion to be transported through the bed of solids.

In general, the method according to the present invention concerns aprocess for treating metal objects to improve their physical properties.The treating steps contemplated involve the transfer of heat to or fromsuch objects in a controlled manner in order to obtain or to modifycertain characteristics in the structure of the metal composing theobject, and in accordance with certain well known basic standards forsuch treatment. According to this method, a metal object to be treatedis immersed in a bath as provided by a bed of fluidized finely dividedor powdered material, such as the bed 2 of FiG. 1, or the bed 49 of FGS.2 and 3. The hed is iiuidized to a degree within a range of notsubstantially less Ithan the level of incipient fluidity, at which levelit is a quiescent fiuidized bed, such as has been defined in Industrialand Engineering Chemistry, Vol. 41, p. 1249, .lune 1949, and notsubstantially more than required to produce a turbulent bed as thereindefined, and below that level at which a bed of such material no longerretains a discernible upper surface and the whole mass of finely dividedsolid material becomes a dispersed suspension in the fiuidizing medium.These levels of fiuidization, of course, are governed by severalfactors, including actual density of the solid material, particle size,and the linear velocity of the gaseous uidizing medium as injected intothe mass or bed of solid material. The factors may be readily determinedand correlated, however, for the purpose of this invention, by anyperson skilled in the art. For example, in the case of relatively fineparticles such as those in the G-400 mesh range, the point of incipientfluidity may be as low as 0.01 ft./sec. superficial linear Velocity(i.e., calculated on the basis of an empty vessel) of fluidizing gaspassing upward .through the powder, while coarser particles such as 6-12mesh may require as much as 1.0 or 2.0 ft./sec. The upper limit offiuidization contemplated will be at a linear velocity of about 5.0 feetper second. Density has a similar effect, heavy materials such as iron,nickel, etc., requiring higher fluidization gas velocities than lightmaterials such as carbon, silica, magnesium, aluminum, etc.

The bed of solid materials, as shown in FIGS. 1, 2, and 3, is fiuidizedby the injection of a gaseous, fiuidizing medium upwardly through themass of materials as by way of the supply conduit 4, manifold conduit 5,and discharge conduits 3 as yshown in FIG. 1, or, as in FIGS. 2 and 3,by way of the pump 57, supply conduit 54, branch lines 55 and themanifold injection nozzles 53. Control of the rate of injection of theiiuidizing medium is obtained by suitable means such as the valves shownin the several supply and branch line conduits, and also by operation ofthe pump S7 of FIGS. 2 and 3.

Further, the metal object is immersed in the bed of fiuidized materialand conveyed therethrough in a series of treating stages, which may ormay not be sharply defined. n each stage heat is added or abstractedfrom the metal object by contact with and by the individual solidparticles. By liuidization of the bed, the particles are maintained insubstantially constant motion, and with the metal object immersed in thebed, the individual particles impinge upon the surface of the object ata rate determined by the degree of fiuidization of .the bed and thedegree of turbulence imparted thereby. Inasmuch as the rate of heattransfer has been found to depend upon the rate or frequency of particleimpact, at zero or incipient fluidity lthe rate of heat transfer is verylow,

and the bed of solid material has a substantial insulating effect, whilewith high fiuidity and the individual particles of the bed in turbulentmotion, the heat transfer rate and also the thermal conductivity of themass of solid material are increased markedly,

As an example of the change in heat transfer between powder and a metalWall as fiuidization is varied, a metal powder having a density of about9.0 and a particle size in the range of about 200 to 400 mesh had athermal con-v ductivity constant of 0.19 B.t.u./(hr.)(sq.ft.)( F./ft.)when unfluidized, a heat transfer coeflicient of 20 B.t.u./ (hr.) (sq.ft.) F.) when fluidized at a superficial linear gas velocity of 0.2.ft./sec., and a heat transfer coefiicient of 46 when fiuidized at avelocity of 2.0 ft./sec. With carbon powder of the same particle sizerange, the heat transfer coefficient was about 12 at 0.05 ft./sec. andabout 3l at 1.2 fL/sec. With carbon powder of 20-48 mesh, the heattransfer coefiicient was about 6 when unuidized by gas at 0.5 ft./sec.,16.5 when fluidized at 1.35 ft./sec. and 30 when ffuidized at 2.4ft./sec. By increasing the turbulence of these powders Still further itwould be possible to increase the heat transfer coefficient to in theneighborhood of or even higher. This method of varying the heat transferrate by varying the fiuidity and turbulence of fiuidized solids isutilized in the present invention to obtain a degree of flexibility anda degree of control in the heat treating of metals that was notobtainable in the prior art. In the method now contemplated, these heattransfer characteristics are employed to control the rate at which themetal objects are heated or cooled while immersed in the bed of solidmaterials provided in either of the vessels 1 or 31.

In order to obtain the desired transfer of heat to or from the metalobject it is, of course, essential to maintain a temperaturedifferential between the object and the fiuidized solid material. Thisis accomplished according tothe present invention in one or more ofseveral ways. For example, in the operation, as carried out in theapparatus of FIG. l, the solid material Vis circulatedl from vessel 1 tovessel l@ by way of the transfer conduit 13. In conduit 13 a fiuid heatexchange medium, such as a hot or cold gaseous medium, is injected intodirect heat exchange relationship to the solid material. By injectingsuch gases in the desired direction of flow, the stream of gas acts tomove the solid particles in the direction of vessel l0. Further, theinjected gases are employed to increase the fluidization ofthe mass ofparticles in the conduit and in the vessel lil, so as to produce a lowerdensity in the mass of material in vessel lil than in the bed ofmaterial in vessel 1, and thereby create gravity circulation between thetwo vessels. The solids circulated through line 13 are maintained incontact with the injected heat exchange medium during passagetherethrough, and through the vessel l0, receiving or giving up heattherein. By suitable control of the rate of circulation any desiredtemperature differential may be established and maintained between themetal object being treated and the bath of finely divided solids invessel 1.

These dierential temperatures between the metal object and the solidmaterial also may be established and maintained, as shown in FIG. 1, bycirculation of a liquid heat exchange medium through the conduits 8 intoindirect heat exchange relation with the bed of solid materials invessel l. In addition, by heating or cooling the gaseous tluidizingmedium introduced through conduits 5, 4 and 3, the desired control ofdifferential temperatures may be further supplemented.

In the apparatus of FIGS. 2 and 3, the desired temperature differentialsbetween the metal object and the solid material may be established andmaintained both by circulation of solids through the chamber 39, and byintroducing heated or cooled ffuidizing gases through the conduit systemincluding conduit 56, pump 57, and conduit connections 5.4, 55, and 53.In chamber 39, heat may be added to the solid material circulatedtherethrough by direct heat exchange either with hot combustion gasesproduced by burning fuel and air in burners 67 or otherwise, aspreviously set forth. In any event, circulation through chambers 39 andi0 is produced, as in the apparatus of FIG. 1, by increased iiuidizationof the solid materials in chamber 39, as compared with that in chamber40. Circulation may be controlled positively by suitable adjustment ofthe plate elements 44 and 45. In fluidizing the materials contained invessel 3l of FIG. 2, the gaseous fluidizing medium is supplied andinjected into the well 36 at a somewhat lower velocity than into thechamber 40, so as to maintain the mass of material at a higher densityin the Well. In this marmer, in combination with the dependent baflie34, a seal or trap is established at the combined entrance and exit ofchamber 4t), such that solid particles which may be entrained by theiiuidizing medium at the higher injection velocities which may exist inchamber 40 during certain portions of the process, may be prevented fromescaping from the vessel, and may be substantially recovered byseparation as in the cyclone separator 61. The gaseous iiuidizing mediumis injected into the well 36 at a rate to maintain iiuidization at theminimum carryover or loss through the open upper end of the wellsubstantially as described with reference to the open upper end of thevessel 1 in FIG. 1.

As previously indicated, the gaseous uidizing medium which is passedthrough the solid materials contained in chamber 4t) of vessel 31 inFIGS. 2 and 3, is vented from the chamber by way of the cycloneseparator 61, wherein solid particles carried over with the uidizingVmedium are separated and returned to the main body by way of the dipleg 62. The gaseous medium itself may be exhausted through the line 63with valve 65 open and valve 66 in conduit connection 64 closed.Alternately, valve 65 may be closed, and valve 66 opened to provide forrecirculation of the vented gas by way of conduit 64 and the pump 57.This mode of operation is particularly contemplated when the gaseousmedium may be rare or expensive, such as hydrogen and dissociatedammonia,

In FIG. 2, the level 52 of the body of iiuidized solid materials isshown as being substantially below the indicated levels 5G and 51 of thesolid materials in Well 36 and chamber 39, respectively. Thesedifferences in levels are exaggerated to some extent to illustrate thetendency of pressure drop through the separator 61 to produce a positivegas pressure in chamber 40, and thereby to depress the level 52 belowthat which may exist in the well 36. Also as indicated above, thedensity of the mass of material in the chamber 39 will be somewhat lessthan that existing in chamber 40. The body of material in chamber 39will thus be expanded by the greater degree of uidization inducedtherein, with a consequent elevation of the surface level. In actualoperation, these levels may not vary, one from another, to such anexaggerated extent as shown.

In the operations contemplated, a variety of gaseous fiuidizing mediamay be employed. The most common of these will be flue gas. Preferablythe flue gas employed will be rich in carbon monoxide or unburnedhydrocarbons in order to avoid diiiiculties from scaling of the metalswhere high concentrations of carbon dioxide, free oxygen or air, sulfurdioxide, and water vapor may be present. In certain treatments, such aswhere a reducing atmosphere is specifically indicated, hydrogen or'dissociated ammonia may be employed alone or in combination with othergases. In other treatments, as in nitriding ferrous metals, theiiuidizing gas may include such gases as hydrogen cyanide or ammoniumcyanide. It is a characteristic of the present invention, that bycomparison with the requirements of the prior art, the volume of gasrequired for operation is relatively small,

.8 and therefore the use of rare and more expensive gaseous mediabecomes economically practical.

The finely divided and powdered solid materials suitable for use as theheat transfer medium according to the present invention may includefinely divided sand, Zirconia, ferro-silicon, silica gel, alumina,bauxite, carbon, coke, brick dust, iron oxide, clay, ground porcelain,powdered or microspherical metals, used powdered silica alumina crackingcatalyst, or any other inert or reactive solid material according to thetreatment contemplated. The particular material which is employeddepends somewhat on the particular heat treating process, since somematerials are relatively inert at low temperature but may cause changesin the surface 0f the metal being treated at high temperatures. Anespecially desirable solid material for heat treating is finely dividedmetal, particularly metal having approximately the same composition asthe metal stock being heat treated. For example, in the heat treating ofcast steel, steel powder having about the same carbon content may beemployed to advantage.

In other cases it is advantageous to employ as the heat treating mediuma material that will cause a change in the surface of ythe metal beingtreated. For example, the present invention is eminently suitable forthe surface carburizing and nitriding of ferrous metals, and for thecementation of various metals. In carburizing, the metal stock is heattreated at a temperature in the range from about 1600 F. to about 1750F. in the presence of a iluidized carburizing material such as hardwoodcharcoal, petroleum coke, metallic carbides such as iron carbide,charred bone, and bituminous coal, to which may be added activators suchas barium carbonate, calcium carbonate and sodium carbonate. Thefluidization gas may be a neutral gas, for example nitrogen, butpreferably it is a carburizing gas such as a petroleum gas. In thelatter case the tluidized solid may be non-carbonaceous if desired. Theadvantages of the invention as applied to carburizing will be evidentfrom a consideration of the prior art stationary process, wherein it wasnecessary to place small metal parts in small treating pots because ofheat gradients, wherein it was necessary to pack the parts uniformlyseparated according to a pattern which varied with size and shape, andwherein it was necessary 'to seal the pots carefully against the adventof furnace gases. The requirements for successful carburizing of ovenheating, careful temperature control within il0 F.

and avoidance of contact with air or furnace gases are easily met withthe iiuidized solid process of the present invention.

Similarly, the present invention is useful in nitriding ferrous metals,for example, by immersing the metal in a fluidized solid such as ironpower or iron microspheres and employing a nitriding gas such as HCN,NH4CN, etc., which may be diluted with other gases if desired. Highmelting cyanides, such as sodium cyanide or potassium cyanide, can alsobe employed as the tluidized solid. The

use of other tiuidized metalsrsuch as aluminum, zinc,

chromium, cadmium, tungsten, vanadium, cobalt, titanium, silicon,zirconium, molybdenum, tantallum, boron, manganese,` and beryllium, atcementation temperatures which may range from about 650 F. to about2550o F.,

together -with relatively inert fiuidizing gas such. as nitrogen,hydrogen, helium, etc., results in the formation `of quite even andtenacious alloy coatings.

Example I-Annealz'ng Steel stock consisting of 3 x l2 bars having acarbon content of 0.33% is transported by conveyor 24 into a vessel suchas designated by the numeral l in FIG. l.

, 9 Y The vessel 1 contains a bed of nely divided solid material such asluidized foundry sand having a particle size of about 80-100 mesh andhaving a temperature of about 800 F. The steel stock is immersed in theluidized solids below the level shown at 7. Flue gas is employed as alluidizing gas for the solids, being introduced through line at a ratesuch that the upward superficial gas velocity in vessel 1 is 1.5ft./sec. and to produce turbulence in the bed. The heat transfercoeliicient under these conditions is about 85 B.t.u./(hr.)(sq. ft.)(E). The temperature of the solids in vessel l is increased byintroducing hot solids from heating vessel via line 15. Injection of hothue gases through conduit 17 etlects circulation of solids from vessel lto heating vessel l0. The gas velocity in heating vessel l0 willnormally be greater than that in vessel 1, and therefore in vessel 1!the density will be less and the level will be higher than in vessel 1.Under these conditions, luidized solids will circulate from vessel 1,through line 13, into heating vessel 10 and thence through line 15 intovessel 1. The rate of solids ow is controlled by the valves 1d and lr6.

When the temperature of the solids and of the steel stock in vessel 1reaches 1500 F., the circulation of solids from heating vessel 10 tovessel 1 is stopped. The steel stock is then subjected to heat soakingat 1500 F. for about 3 hours, about one hour or" soaking time beingallowed for each inch of cross section yof the stock. During the heatsoaking period, it is desirable to -l aintain the steel stock at theheat soaking temperature, and in order to reduce the loss of heat fromthe steel stock the uidizing gas rate in Vessel l is reduced to fromabout 0.05 to 0.1 ft./sec. superficial linear velocity, producing a lessturbulent condition in the bed than in the initial stage of treatment.Under these conditions, the heat transfer coecient is about 3-5 B.t.u./(hr.) (sq. ft.) F. At the end of the soaking period, the temperature ofthe uidized solids in vessel 1 and of the steel stock irnmersed thereinis reduced slowly over a period of 4-5 hours. The temperature isdecreased by passmg a cooling medium such as water through cooling coils3 in Vessel i. During the cooling period, the uidizing gas velocity andthereby turbulence in the bed is increased to about 0.5 ft./sec., givinga heat transfer coeliicient or about 50 B.t.u./(hr.)(sq. ft.)( E), inorder to increase the rate of heat transfer from the steel stock to theiluidized solids and from the uidized solids to the cooling coils ti.When the temperature reaches 800 F. the annealed steel stock is removedfrom the tluidized bed by conveyor 2dand is allowed to cool further inair.

Example II-Hardenz'ng and Quel/:cking Steel gears are heated to 1450 F.in a vessel such as vessel 1 of FIG. l, substantially as described forthe heating period of Example I. They are then removed from the heatingvessel as by the mechanical conveyor 24, and are transported to a secondsimilar vessel which contains a bed of fluidized iron powder 50-100 meshat a temperature of about 100 F. The gears are there immersed below thelevel of the fluidized iron. Flue gas is used to fluidize the ironpowder, the gas rate being about 1.3 ft./sec. superiicial linearvelocity. Under these conditions the bed is in a substantially turbulentcondition, and the heat transfer coeticient is about 80 B.t.u./ (hr.)(sq. ft.) F.) and the gears are rapidly quenched to a temperature ofabout 750 F. Depending on the dimensions and volume of the gears beingtreated, the time of quenching may vary from less than a minute to -60minutes. At this point, the uidizing gas rate is decreased sharply toabout 0.05 to 0.35 ft./sec. in order to decrease the cooling rate andallow the hardening transformation to take place. Under theseconditions, the heat transfer coetiicient is about 3-30 Btu/(hr.) (sq.ft. F.). When the gears have reached a temperature of about 150 F., theyare removed from the fluidized solids by the conveyor and can betempered immediately.

sasso l0 VExample III--Tempering The steel gears hardened as in ExampleII are transported by a conveyor to another vessel such as shown in FIG.l. The gears are immersed below the level of iiuidized solids containedin vessel 1, the solids being -400 mesh spent silica-alumina crackingcatalyst obtained as a by-product in the petroleum industry. Thetemperature of the luidized solids in vessel 1 is about 200 F., heatbeing supplied by het solids circulated from heating vessel l0 as wasdescribed in Example I. Flue gas relatively free of acidic gases Vandcontaining a relatively high proportion of carbon monoxide is suppliedas iiuidizing gas through line 5 at a rate equivalent to a superficialgas velocity of about 0.30 ft./sec., giving a heat transfer coeflicientof about 40-60 B .t.u./ (hr.) (sq. ft.) l). When the temperature of thegears approaches 800 F., generally in 1/2-2 hours depending on the sizeof the gears, circulation of hot solids from heater l0 is stopped andthe fiuidizing gas rate is decreasd to a low level in order to decreaseheat transfer and provide a soaking period of about two hours. The gasrate during the soaking period is preferably at or near the minimumfluidizing gas velocity, for example, in the range of about 0.01 to 0.05ft./sec., where the heat transfer coefcient is in the range of about 2to l0 B.t.u./ (hr.) (sq. ft.) F). At the end of the soaking period, thegas rate is increased to about 0.15 to 0.25 ft./sec. in order toincrease turbulence and thereby the heat transfer rate. Water is passedthrough cooling coils 8, and the uidized solids and the steel gearsimmersed therein arel slowly cooled to about 200 F., whereupon thetempered gears are removed from the iiuidized solids.

Example lV-Carburzz'ng An alloy steel part to be case hardened isintroduced into the vessel l as shown in FIG. l. Vessel 1 contains a10G-300 mesh iluidized solid carburizing agent, com' prising hardwoodcharcoal, petroleum coke and barium carbonate, at a temperature of about200 F. The solids are luidized by gas introduced through line 5. Theiluidizing gas is a hydrocarbon gas, such as propane, containing adiluent gas such as carbon monoxide. The gas velocity is in the range ofabout 0.75 to 1.5 ft./ sec. After the steel stock is introduced belowlevel 7, the temperature of the solids is increased to about l700 F. bycirculating through the vessel l0, with heat being supplied to thesolids in heater l0 by hot ue gas of a reducing nature. When thetemperature of the steel stock approaches 1700 F., the liuidizing gasrate is decreased to a value in the range of about 0.1 to 0.3 ft./sec.and circulation from vessel t0 is stopped. The steel stock is then heatsoaked at 1700 F. for about 4 6 hours, which gives a carburized casedepth of from about 0.045 to about 0.060 inches. Cooling huid is thenpassed through coils S and the uidizing gas rate is increased to a valuein the range of about 0.75 to l.5 ft./sec. stock reaches l450 F., it isremoved from the carburizing agent and is subjected to a quenchingoperation as described in Example il.

Example V-Sherrzrczing Sheet iron plates to be coated with zinc bysherardizing are immersed in a tiuidized mixture comprising equal partsby weight of zinc powder and zinc oxide powder having a particle size inthe range of about 80 to 300 mesh. The powder is contained in vessel )las shown in FIG. 1 and is tluidized by nitrogen passing therethrough ata superficial Velocity of 1.2 ft./sec. The powder in vessel 1 is thenheated to a temperature of 700 F. by circulating the powder throughheating vessel 10 in heat exchange relation to hot ue gas of a reducingnature. When the powder in vessel i reaches a temperature of 700 F.,powder circulation is stopped and the uidizing gas rate is decreased tosubstantially zero. After the sheet iron plates have undergone a heatsoaking period of about three hours the When the temperature of thesteel Y combined inlet and outlet.

fluidizing gas rate is restored to its former level, `cooling water isadmitted to coils 8, and the temperature of the powder and of the steelplatesis reduced to ambient temperature. The coated plates are thenremoved from vessel l, and a fresh batch of plates is introducedthereto.

The apparatus, as shown in FIG. 2, is particularly adapted to employmentwherethe metal object to be treated must be passed through a series oftreating stages in succession. The apparatus is most suitable for anoperation such as an annealing operation, wherein the metal object isrequired to pass through a heating stage in Awhich it is graduallyraised to an annealing temperature, held at such temperature for apredetermined period, and then gradually cooled before removal from thetreating vessel. In the apparatus as shown, it is possiblesimultaneously to accommodate metal objects in each of the treatingstages indicated.

, In the apparatus as shown the treating stages are in substantiallycontinuous sequence. This is made possible by suitable control ofinjection of the fiuidization medium into the body of solid material inthe chamber 40, and by establishment of a suitable temperature gradientthrough the mass of solid material in the chamber, from the partition 37to the bale 34.

In an annealing operation, for example, finely divided solids such asfoundry sand of a particle size of about 80 to 100 mesh may becirculated from and to chamber 40 through the chamber 39 in the mannerpreviously described. In chamber 39, the sand is heated and uidized bycombustion gas produced by burning a fuel gas and air in burners 67. Bysuitable control of the heating and circulation of sand a temperaturegradient may be established in the bed ranging from about 800 adjacentthe baffle 34 to about 1500 adjacent the partition 37. With suchtemperatures, a metal object such as the steel bars of Example I abovemay be introduced into chamber i0 'through the .Well 36 by means ofconveyor 73. As the steel stock is passed through the travel path alongone side of bathe 41 from bale 34 to partition 37, ue gas is introducedas a fluidizing medium by way of the conduit 54, branch lines and 4theinjection nozzles 53.

By means of the valves in branch lines 55, this gas is injected throughthe nozzles along the path so as initially to produce a superficial gasvelocity in the initial stage in the neighborhood of about 1.5 ft./sec.At such rate of injection, the rate of heat transfer between the metalstock and the finely divided sand is high, the sand having a heattransfer coefficient of about 85 B.t.u./ (hr.) (sq. ft.) F.). Then asthe metal stock progresses along the travel path and reaches atemperature of about l500 F., in that area the rate of injection of thefluidizing medium is reduced to from about 0.05 to about 0.1 ft./sec.during further progress of the metal stock as across the face ofpartition 37, and into the return travel path along the opposite side ofbaie 41and continuing injection at suchV rate for a period of about 3hours to obtain the desired temperature throughout the stock. In thisstage, and at such rates of injection of the iuidizing medium, the heattransfer coefficient of the solids is reduced to from about 3 to 5B.t.u./ (hr.) (sq. ft.) E). By suitable regulation of the conveyorspeed, this may be accomplished as the stock enters the return travelpath, and in this area the liuidizing gas velocity is increased to about0.5 ft./sec., increasing the heat transfer coeiiicient of the sand inthis area to about 50 Blu/(hr.) (sq. ft.) R). Preferably, in the areaimmediately adjacent the bafe 34, the rate of injection of the uidizinggas is again increased, as to substantially the rate of injection in theinitial stage. At this point, the spacing of baie 41 from the baiiie 34permits Acirculation of the solid material around the baille 41 such asto aid in heat recovery and heat equalization at the The stock is thenremoved from the chamber 40 through the well 3f by further `progress o fthe conveyor. In the well 36, the fiuidizing was l2 gas is injected at arate below that which may exist in adjoining portions of the chamber 40.

Further in accordance with the method as has been set forth above,surface conditioning of the metal objects is also accomplished to removeaccumulations of scale, rust or other undesirable surface deposits andcoatings. Such surface conditioning is accomplished substantially as aresult of the metal objects scouring action of the nely divided solidparticles when they lare set in motion by fiuidization and impingeagainst the metal objects during treatment thereof.

What is claimed is:

l. The method of altering the surface of a ferrous metal object whichcomprises introducing a gas to a vessel containing a multiplicity offinely-divided carbonaceous solid particles substantially incombustibleunder the conditions of the treatment and reactive with the surface ofsaid ferrous metal object at a rate suitable to uidize said solidparticles, introducing a ferrous metal object into the said vessel andlsubmerging it in said tluidized solid particles at a region spaced fromsaid vessel, maintaining the temperature of said fluidized solidparticles at a temperature between about lai-50 F. and l750 F. andthereafter withdrawing said ferrous metal object from said vessel.

2. Process according to claim 1 in which the finely divided solidmaterial is a metal cyanide.

3. A process for altering the surface of ferrous metal objects whichcomprises maintaining in a substantially confined treating zone a bed ofnely divided carburizing solid particles, passing a gaseous fiuidizingmedium containing petroleum gas upwardly through said treating zone at acontrolled velocity to tiuidize said nely divided carbun'zing solidparticles and to impart motion to the individual solid particles in saidbed, introducing a ferrous metal object to be treated into said treatingzone and substantially completely submerging said ferrous metal objectin said bed of nely divided fluidized carburizing solid particleswhereby said ferrous metal object within said treating zone is subjectedto direct impinging Contact of individual particles of said finelydivided uidized solid particles in motion, selecting an elevatedtemperature for said bed of solids and continuing the treatment for aperiod of time sufficient to accomplish the desired alteration in thesurface of said ferrous metal object, heat soaking said ferrous metalobject for an extended period of time in said fluidized solid particlesand then cooling and withdrawing said treated ferrous metal object fromsaid treating zone.

4. A process for altering the surface of individual metal objects whichcomprises maintaining in a substantially conned treating zone a bed offinely-divided solid particles chemically reactive with the surface ofsaid metal objects under selected temperature conditions, passing agaseous medium upwardly through said treating zone to iluidize saidfinely divided particles, introducing a ferrous metal object to besurface treated into said treating zone and submerging said ferrousmetal object in said bed of finely divided uidized solid particles,selecting an elevated temperature for said tluidized bed of -solids andselecting the time of contact between said ferrous metal yobject and thetiuidized solid particles to effect chemical reaction between thesurface of said ferrous object and said solid particles to kalter thesurface of said ferrous metal object, and then withdrawing the surfacetreated ferrous metal object from said treating zone.

5. A process according to claim 4 wherein the surface of said ferrousmetal object being treated is carburized and thercarbon content of thesurface of said metal object is increased.

6. A process according to claim 4 wherein said surface treating of saidferrous metal object is effected in the presence of a cyanide compound.

7. A process according to claim 6 wherein nitriding of the.v surface Aofsaid ferrous. metal object is effected.

8. A method of treatingferrous metal objects to effect a change in thesurface thereof which comprises submerging a ferrous metal object in abed of finely divided solids, maintaining said finely divided solids ina fluidized condition by passing a carburizing gas upwardly through saidbed of solids, selecting a temperature of above about 1600 F. for saiduidized bed and selecting the time of contact between said ferrous metalobject and said carburizing gas to effect chemical reaction between saidferrous metal object and said carburizing gas in the presence of saidsolid particles of said iiuidized bed and removing the surface treatedferrous metal object from said bed.

9. A process according to claim 8 wherein said carburizing gas comprisesa hydrocarbon gas and said solids comprise non-carbonaceous solids.

10. A method for case hardening steel objects which comprisesmaintaining a bed of finely divided carburizing solids, passing agaseous medium upwardly through said bed of solid particles at avelocity controlled to maintain said solid particles in a iiuidizedstate, submerging a steel object to be case hardened in said bed oftluidized solid particles heated to a temperature between about 1600 F.and l750 F. whereby said steel object is subjected to impinging contactwith said fluidized solid carburizing particles, regulating thetemperature of said uidized bed and the time of Contact between saidsteel object and said fluidized solid carburizing particles to effectreaction between said steel object and said carburizng solids to caseharden said steel object, cooling said steel object and thereafterremoving said case hardened steel object from said fluidized bed ofsolids.

11. A process for sherardizing ferrous metal objects which comprisesmaintaining a bed of finely divided solid particles comprising zincpowder and zinc oxide powder in a substantially conined treating zone,passing an inert gaseous medium upwardly through said bed of solidparticles at a velocity controlled to maintain said solid particles in aliuidized state, immersing a ferrous metal object to be sherardized insaid bed of liuidized solid particles whereby said ferrous metal objectis subjected to impinging contact with said uidized particles, selectinga temperature of said uidized bed of solid particles and the time ofcontact between said ferrous metal object and fluidized particles toeffect the desired extent of sherardizing and thereafter removing saidtreated ferrous metal object from said treating zone.

12. The process according to claim 11 wherein the selected temperatureof said fluidized bed is about 700 F., said ferrous metal object issoaked in said confined treating zone for an extended period of time andthereafter cooled and removed from said confined treating zone.

13. A process for coating a ferrous metal object with another metalwhich comprises maintaining a bed of nely divided metal other than saidferrous metal object in a treating zone, passing a relatively inertnon-oxidizing gaseous medium upwardly through said bed of metalparticles at a velocity controlled to maintain said metal particles in atiuidized state, immersing said ferrous metal object to be coated insaid bed of tiuidized metal particles whereby/,said ferrous metal objectis subjected to impinging contact with said fluidized metal particles,selecting a cementation temperature of said uidized bed of metalparticles between about 650 F. and 2550 F. and regulating the time ofcontact between said ferrous metal ob-V ject and the fluidized metalparticles to effect the desired coating of said other metal on saidferrous metal object to form a tenacious coating on said ferrous objectand thereafter removing said coated ferrous metal object from saidtreating zone.

References Cited by the Examiner UNITED STATES PATENTS 414,122 10/89Roberts 148-14 489,194 1/93 Mustin 148-14 2,393,909 1/ 46 Johnson. 2,459,83 6 1/ 49 Murphree. 2,509,866 5/50 Hemminger. 3,053,704 9/ 62 Munday14S- 20.3

OTHER REFERENCES The Metals Handbook, 1948 Edition, American Society forMetals, Cleveland, Ohio, pages 677-702, and 712-716 relied on.

DAVID L. RECK, Primary Examiner. WINSTON A. DOUGLAS, Examiner.

1. THE METHOD OF ALTERING THE SURFACE OF A FERROUS METAL OBJECT WHICHCOMPRISES INTRODUCING A GAS TO A VESSEL CONTAINING A MULTIPLICITY OFFINELY-DIVIDED CARBONA-CEOUS SOLID PARTICLES SUBSTANTIALLY INCOMBUSTIBLEUNDER THE CONDITIONS OF THE TREATMENT AND REACTIVE WITH THE SURFACE OFSAID FERROUS METAL OBJECT AT A RATE SUITABLE TO FLUIDIZE SAID SOLIDPARTICLES, INTRODUCING A FERROUS METAL OBJECT INTO THE SAID VESSEL ANDSUBMERGING IT IN SAID FLUIDIZED SOLID PARTICLES AT A REGION SPACED FROMSAID VESSEL, MAINTAINING THE TEMPERATURE OF SAID FLUIDIZED SOLIDPARTICLES AT A TEMPERATURE BETWEEN ABOUT 1450*F.AND 1750*F. ANDTHEREAFTER WITHDRAWING SAID FERROUS METAL OBJECT FROM SAID VESSEL.